The invention relates to making a casing-and-diverging-portion unit for use in particular in making components for rocket exhaust nozzles or outlet cones.
That type of part comprises an internal portion referred to as the “diverging portion”, that is required essentially to present good ability at withstanding temperature and ablation since it is subjected directly to the flow of hot gas. The part also has an external portion, referred to as its “casing”, having the main function of imparting good stiffness to the part.
Particularly because of the specific properties that the diverging portion and the casing need to present, these two portions are made separately. Thereafter they are united to form a casing-and-diverging-portion unit.
A method presently in use for forming composite diverging portions or casings consists in draping and molding plies of carbon fabric preimpregnated with a resin, e.g. a phenolic resin. More precisely, the method comprises draping plies of preimpregnated fabric in a rosette pattern on a mold presenting the shape of the unit to be made, compacting the plies in a vacuum by means of a diaphragm, installing a female mold if necessary, polymerizing the unit in an autoclave or a press, and unmolding the part. This produces a rigid part with fiber reinforcement.
There also exist other known methods of making composite diverging portions or casings such as methods comprising, for example, filamentary winding, braiding, or knitting sleeves.
Nevertheless, with those methods of fabrication, the number of components needed for making the casing-and-diverging-portion unit is relatively large, as is the time required, thus leading to a manufacturing cost that is high.
Furthermore, since the diverging portion and the casing are made separately, assembling these two elements together can be complex and can present difficulties of implementation. For example, aligning the contact surfaces between the casing and the diverging portion requires additional machining, thereby increasing the time and the cost of fabrication accordingly. In addition, bonding the two elements together requires the assembly surfaces to be prepared and good control over the adhesive used in order to obtain good adhesion at all points.
In addition, the above methods are difficult to implement. In particular, draping fiber plies to form the fiber reinforcement is a manual operation that is performed directly on a shaping support and that does not enable accurate control to be maintained over the orientation and the quantity of fibers at all points in the reinforcement.
Furthermore, most existing methods use preimpregnated fibers that are less flexible than dry fibers, thereby making it even more difficult to shape the plies to match the shape of a body of revolution such as a diverging portion and the associated casing.